mcc_parse: save the last_timecode to have a fallback

If the current timecode can't be parsed, the plugin falls back
to last knonw timecode to at least have something. The code
that does this already existed but the last_timecode was just
never set.

gst-plugin-closedcaption/src/mcc_parse.rs

@@ -434,6 +434,9 @@ impl MccParse {
 
         state.add_buffer_metadata(element, &mut buffer, &timecode, &framerate);
 
+        // Update the last_timecode to the current one
+        state.last_timecode = Some(timecode);
+
         // Drop our state mutex while we push out buffers or events
         drop(state);